A composite separation membrane is produced by laminating a separation functional layer having desired separation ability on a porous support. As the separation functional layer, a membrane of a material selected from organic compounds such as polyamide, polysulfone and cellulose acetate is used in accordance with its intended purpose. In the field of reverse osmosis membranes, it is known that a polyamide membrane obtained by polymerization of amine and acid halide is suitable as a separation functional layer. This polyamide membrane is typically an aromatic polyamide membrane obtained by interfacial polymerization of a polyfunctional aromatic amine and a polyfunctional aromatic acid halide. As the porous support for supporting the polyamide membrane, a support in which a microporous layer of polysulfone or the like is formed on a porous supporting material such as a nonwoven fabric is commonly used.
It is known that many microscopic projections are formed on the surface of a polyamide membrane obtained by interfacial polymerization on a porous support. There have been attempts to optimize this microstructure of the surface based on the observation with a scanning electron microscope (SEM) or the measurement with an atomic force microscope (AFM) so as to improve the performance of the composite separation membrane.
Patent Literature 1 discloses a polyamide membrane having an average surface roughness Ra of 55 nm or more. According to Patent Literature 1, an increase in the surface roughness of the polyamide membrane results in the desired improved separation performance required for a reverse osmosis membrane.
Patent Literature 2 discloses a polyamide membrane with projections formed on its surface and having an average diameter of 150 nm or less in terms of equivalent circle diameter. An SEM micrograph in Patent Literature 2 shows that many fine projections are formed almost uniformly on the surface of the polyamide membrane. According to Patent Literature 2, finer projections on the surface of the polyamide membrane results in the improved performance of a composite separation membrane used at high pressure.
Patent Literature 3 discloses a polyamide membrane (separation functional layer) having a parameter L/T of 50 or more when the thickness of this layer is T μm and the actual length of this layer per unit length of the surface is L μm. As shown in FIG. 1 of Patent Literature 3, this separation functional layer is deeply corrugated to form projections and extends over the porous support. According to Patent Literature 3, a separation functional layer satisfying the above conditions has an increased surface area relative to its thickness, resulting in the improved separation performance of the membrane.
Conventional polyamide membranes (separation functional layers) as disclosed in Patent Literatures 1 to 3 are each a single layer formed on the surface of a porous support, and this layer has many projections on the entire surface thereof.
As a specific means for improving the separation performance of a membrane, an addition of an organic compound such as an alcohol or an ether to a solution for forming a polyamide membrane is known (Patent Literatures 1 and 3). Patent Literature 4 discloses many examples of membranes obtained by adding various additives to measure their separation performance (Tables 1 to 3). The use of an additive is effective in increasing the permeation flux of a membrane. Patent Literature 4 reports that the highest permeation flux was obtained in the production examples in which isopropanol was added to each of two solutions used to form a polyamide membrane (Table 1 and Examples 3 to 5).
Composite separation membranes are required to have further improved separation performance. Particularly in recent years, they are required to have improved separation performance suitable for applications, such as wastewater treatment and pretreatment for desalination of seawater, which require a high permeation flux more preferentially than a high salt rejection rate. In addition, a growing demand for increasing the permeation flux to reduce the operational energy (that is, for energy saving) has emerged.